Capacitors for use in electronic instruments such as portable telephone and personal computer are demanded to have a small size and a large capacitance. Among these capacitors, a tantalum capacitor is preferred because of its large capacitance for the size and good performance. In this tantalum capacitor, a sintered body of tantalum powder is generally used for the anode moiety. In order to increase the capacitance of the tantalum capacitor, it is necessary to increase the weight of the sintered body or to use a sintered body increased in the surface area by pulverizing the tantalum powder.
The former method of increasing the weight of the sintered body necessarily involves enlargement of the capacitor shape and cannot satisfy the requirement for downsizing. On the other hand, in the latter method of pulverizing tantalum powder to increase the surface area, the pore size of the tantalum sintered body decreases or closed pores increase at the stage of sintering and therefore, impregnation of the cathode agent in the later process becomes difficult.
As one of means for solving these problems, a capacitor using a sintered body of powder of materials having a dielectric constant larger than that of tantalum is being studied. Niobium is known as a material having such a large dielectric constant.
The capacitor (niobium capacitor) manufactured using a niobium powder as a raw material is considered, however, inferior in the voltage resistance characteristics to capacitors manufactured using a tantalum powder as a raw material. Furthermore, when the average particle size of niobium powder as a raw material of niobium capacitors is reduced so as to increase the capacitance, a cathode agent cannot be easily impregnated in the later process, similarly to the case of tantalum powder.
With respect to metals such as niobium, JP-A-6-25701 (U.S. Pat. No. 5,407,458) (the term “JP-A” as used herein means an “unexamined published Japanese patent application”) discloses a fine metal powder having a particle size of 1.0 nm to 3 μm, however, this patent publication is silent on the shape (circular degree) of the fine powder, the agglomerated powder of the fine powder and the sintered body for capacitors using the powder.